Machine learning assets are not binary "open" or "closed". There is a continuum of openness.
To make a really poor analogy, this repo is like a version of Linux that you can't cross-compile or port.
To make another really poor (but fitting) analogy, this is like an "open core" SaaS platform that you know you'll never be able to run the features that matter on your own.
This repo scores really low on the "openness" continuum. In this case, you're very limited in what you can do with Chatterbox TTS. You certainly can't improve it or fit it to your data.
> You can fine-tune the weights yourself with your own training code.
This will never be built by anyone, and they know that. If it could be, they'd provide it themselves.
If you're considering Chatterbox TTS, just use MegaTTS3 [1] instead. It's better by all accounts.